The human spine is made up of a column of thirty-three bones and their adjoining structures. The vertebrae near the head are known as the presaccral vertebrae which are separate bones capable of individual movement. The bodies of these vertebrae are connected by anterior and posterior ligaments and by disks of fibrocartilage generally known as intervertebral disks. These disks are positioned between opposite faces of adjacent vertebral bodies. This column of vertebrae and intervertebral disks form a central axis that supports the head and torso. These vertebrae also enclose an opening through which the spinal cord passes therebetween.
The presaccral vertebrae are normally held in position to one another by the intervertebral disks, ligaments and musculature of the body. These vertebrae move relative to adjacent vertebrae thus permitting the head to be turned relative the body and providing a wide range of flexibility to the spine.
One of the most costly health problems to society involves back pain and pathology of the spine. These problems can affect individuals of all ages and can result in great suffering to victims. Back pain can be caused by several factors such as congenital deformities, traumatic injuries, degenerative changes to the spine and the like. Such changes can cause painful excessive motion, or collapse of a motion segment resulting in the contraction of the spinal canal and compresses the neural structures causing debilitating pain, paralysis or both which in turn can result in nerve root compression or spinal stenosis.
Nerve conduction disorders can also be associated with invertible disks or the vertebrae themselves. One such condition is herniation of the intervertebral disk, in which a small amount of tissue protrudes from the sides of the disk into the foramen to compress the spinal cord. A second common condition involves the development of small bone spurs, termed osteophytes, along the posterior surface of the vertebral body, again impinging on the spinal cord.
Upon identification of these abnormalities, surgery may be required to correct the problem. For those problems associated with the formation of osteophytes or herniations of the intervertebral disk, one such surgical procedure is intervertebral diskectomy. In this procedure, the involved vertebrae are exposed and the intervertebral disk is removed, thus removing the offending tissue, or providing access for the removal of the bone osteophytes. A second procedure, termed a spinal fusion, may then be required to fix the vertebrae together to prevent movement and maintain a space originally occupied by the intervertebral disk. Although this procedure may result in some minor loss and flexibility in the spine, due to the relatively large number of vertebrae, the minor loss of mobility is typically acceptable.
During a spinal fusion following a diskectomy, a prosthetic implant or spinal implant is inserted into the intervertebral space. This prosthetic implant is often a bone graft removed from another portion of the patient's body, termed an autograph. The use of bone taken from the patient's body has the important advantage of avoiding rejection of the implant, but has several shortcomings. There is always a risk in opening a second surgical site in obtaining the implant, which can lead to infection or pain for the patient, and the site of the implant is weakened by the removal of bony material. The bone implant may not be perfectly shaped and placed, leading to slippage or absorption of the implant, or failure of the implant to fuse with the vertebrae.
Other options for a graft source of the implant are bone removed from cadavers, termed allograft, or from other species, termed a xenograft. In these cases while there is the benefit of not having a second surgical site as a possible source of infection or pain, there is increased difficulty of the graft rejection and the risk of transmitting communicable diseases.
An alternative approach is using a bone graft or to use a manufactured implant made of a synthetic material that is biologically compatible with the body and the vertebrae. Several compositions and geometries of such implants have been utilized, ranging from simple blocks of material to carefully shaped implants, with varying success.
There has been an extensive number of attempts in developing an acceptable prosthetic implant that can be used to replace an intervertebral disk and yet maintain the stability of the intervertebral disk spaced between adjacent vertebrae, at least until complete arthrodesis is achieved. These prosthetic implants have taken many forms. While many types of synthetic prosthetic devices have been proposed, the success ratio has been low and the surgical procedures have been complicated and often traumatic to the patient.
One of the more prevailing designs of these prosthetic implants takes form of a cylindrical implant. These types of prosthetic implants are represented by Brantigan U.S. Pat. No. 4,878,915 and Ray U.S. Pat. No. 4,961,740. In these cylindrical implants, the exterior portion of the cylinder can be threaded to facilitate insertion of the prosthetic device. Some of these prosthetic implants are designed to be pounded into the intervertebral disk space and the vertebral end plates. These types of devices are represented in Brantigan U.S. Pat. No. 4,834,757 and Brantigan U.S. Pat. No. 5,192,327. The Brantigan and Ray patents all disclose prosthetic implants wherein the transverse cross-section of the implant is constant throughout the length of the implant and is typically in the form of a right circular cylinder.
Other prosthetic implants have been developed that do not have a constant cross-section. For instance, the patent to McKinna U.S. Pat. No. 4,714,469 shows a hemispherical implant with elongated protuberances that project into the vertebral end plate. The implant of Bagby U.S. Pat. No. 4,934,848 is in the form of a sphere which is positioned between the centrums of the adjacent vertebrae.
The various prosthetic implants can be generally divided into two basic categories, namely, solid implants and implants that are designed to encourage bone ingrowth. Solid implants are represented by U.S. Pat. Nos. 4,878,915 and 4,349,921. The remaining patents discussed above include some aspect that permits bone to grow across the implant. It has been found that devices which promote natural bone ingrowth achieve a more rapid and stable arthrodesis. These implants are typically filled with autologous bone prior to insertion into the intervertebral disk space. These implants typically include apertures which communicate with openings in the implant, thereby providing a path for tissue growth between the vertebral end plate and the bone or bone substitute within the implant. In preparing the intervertebral disk space for a prosthetic implant, the end plates of the vertebrae are preferably reduced to bleeding bone to facilitate tissue growth within the implant.
A number of difficulties still remain with the many prosthetic implants currently available. While it is recognized that hollow implants which permit bone ingrowth in the bone or bone substitute within the implant is an optimum technique for achieving fusion, most of these devices have difficulty achieving this fusion, at least without the aid of some additional stabilizing device, such as a rod or plate. Moreover, some of these devices are not structurally strong enough to support the heavy loads applied at the most frequently fused vertebral levels, mainly those in the lower lumbar spine.
There has been a need for providing a prosthetic implant that optimizes the bone ingrowth capabilities and strong enough to support the vertebrae until arthrodesis occurs. There is a further need for such an implant that is capable of maintaining or restoring the normal spinal anatomy at the instrumented segment. There is also a need for an implant that exhibits reduced slippage when inserted between vertebrae and diminishes the occurrence of nerve pinching.